This invention relates to an automatic thread tension device of a sewing machine, which determines proper tension of upper and lower threads in response to stitching conditions such as a fabric to be sewn, the upper and lower threads and the tension of the lower thread.
An existing sewing machine is manually adjusted by a machine operator by means of an upper thread tension device in accordance with the stitching conditions such as the type of fabric, threads, patterns, feeding, stitching, lower thread tension, etc. The purpose for the adjustment is to regulate crossing points of the upper and lower threads in the thickness of the fabric under stitching.
There have been many proposals for automating the above mentioned regulation. However, such proposals are concerned with easily distinguishable patterns, feeding or stitching. Since characteristics of the fabrics and the threads vary, they are not completely distinguishable. Therefore, practical and useful proposals have not yet been realized.